My pen pal Cesar sends me the occaisonal triangular folded letter. It's always one sheet, folded unto itself and fairly secure. He adds the 20 cents extra postage for special handling. I thought it was just something he did that was like passing a note in school. Now, I think he might be doing it because of письма-треугольники, or “triangular letters”, the main form of soldier correspondence in Russia during and after World War 2.
The letters were able to be sent without postage and the one sheet design allowed them to use any sort of scrap paper they could put their hands on. There are examples of letters sent on packaging, advertisements, cigarette papers and newspapers. They were forbidden from sealing the triangular letters in any way and the design allowed for easy opening by the censors.
You can read more about triangle letters on this blog which has tons of photos. It also has a handy guide for folding your own triangular mail. My favorite image on the site is the 65th Anniversary issue from 2010. The government distributed them to veterans of the war. They were still free to mail without a stamp anywhere in the country.